Power efficiency is important in high power radio frequency (RF) amplifiers especially for reasons of thermal management and, typically in portable equipment, battery life.
In high power RF amplifiers it is common to use highly non-linear amplifiers in order to achieve efficiency. Such power amplifiers (PA) are commonly termed output stages. In simple terms, if an active device used in an output stage is operated in a linear region then it may have significant heat dissipation and hence relatively poor efficiency and resultant thermal issues. Conversely, if the active device is used essentially as a binary (two-state) switch then it dissipates little power in either on or off state and the only significant power consumption occurs during transitional periods between states. Good efficiency thus dictates fast switching. Energy recovery circuits are used to provide waveform shaping for spectral purity while maintaining efficiency according to well known techniques such as the simple and effective tank circuits used since the early days of vacuum tube amplifiers.
As is well known, the use of non-linear amplifiers works well with constant envelope modulation techniques such as frequency shift keying (FSK), frequency modulation (FM) and other forms of angle modulation.
The use of non-linear active devices for envelope-modulation signals such as amplitude modulation (AM), quadrature AM (QAM), or single sideband AM (SSB) is not so straightforward. Techniques such as corrective and adaptive pre-distortion, feedforward linearization, envelope elimination and restoration (EER) using the Kahn technique, linear amplification with non-linear components (LINC), and Cartesian feedback are each successful but are typically complex and expensive. In previous implementations for consumer grade (i.e. inexpensive) products, overall linearity has been achieved by sacrificing energy efficiency and biasing active devices into a linear region to build linear amplifiers.
Thus, a need exists for an inexpensive method or means for generating high power envelope-modulated radio frequency signals.
Accordingly, the invention presents an inexpensive method and means for generating high power envelope-modulated radio frequency signals.
According to an embodiment of the invention an analog signal, which may be a baseband signal, becomes envelope-modulated upon a high power RF carrier. Further, the creation of envelope modulation by pulse deletion may be impressed, not merely onto a pure carrier, but upon an RF signal that may be modulated with an essentially constant envelope form of modulation. For example, envelope modulation by pulse deletion may be applied to a signal containing angle modulation such as quadrature phase shift keying (QPSK). Thus a separate and additional channel of information may be passed as envelope modulation through a system that formerly carried only angle modulation signals without material degradation of those angle modulated signals.
According to a still further aspect of the invention, control signal may be derived by recovery from a low-level envelope modulated RF signal. Thus, inventive output stages may be fed by a demodulator or similar to create an EER amplifier system. Such an EER amplifier system may, but need not, provide for envelope feedback control.